1. Field of the Invention
The present invention relates to an optical information recording medium, in particular to an optical information recording medium of a write once read many type, in which the recording and reproduction of information are carried out by changing the optical characteristics of a recording material for the optical information recording medium, such as transmittance and reflectivity, with the application of an optical beam thereto.
2. Discussion of Background
The wavelength of a laser used for a currently employed disk system of a write once read many type, which may be referred to as WORM disk system is in the range of 770 nm to 790 nm, and a recording medium for the disk system is constructed in such a manner that recording and reproduction can be performed in the above-mentioned wavelength range.
Sooner or later, it will become inevitable that the capacity of such recording media has to be significantly increased in accordance with the amount of information to be handled. It will also become inevitable that the wavelength of the laser beam used for the recording and reproduction has to be shortened.
Currently many types of optical disks of the write once and read many type for recording and reproduction of data, using cyanine dyes or phthalocyanine dyes as recording materials (hereinafter referred to as WORM optical disks), have been proposed.
For example, WORM optical disks using as a recording material a cyanine dye are disclosed in Japanese Laid-Open Patent Applications 57-82093, 58-56892, 58-112790, 58-114989, 59-85791, 60-83236, 60-89842, and 61-25886. Furthermore, WORM optical disks using as a recording material a phthalocyanine dye are disclosed in Japanese Laid-Open Patent Applications 61-150243, 61-177287, 61-154888, 61-246091, 62-39286, 63-37791, and 63-39888. However, there has not yet been developed a recording material which has high light resistance and excellent preservation stability, and by which recording and reproduction can be performed by an optical pick-up using a laser with a wavelength of 700 nm or less.
Currently employed CD-R (Recordable) disk systems are also constructed in such a manner that recording and reproduction can be performed by a laser beam with a wavelength in the range of 770 nm to 790 nm.
As in the case of the above-mentioned WORM disk system, it will become inevitable that the capacity of the recording media has to be significantly increased in accordance with the amount of information to be handled, and that the wavelength of the laser beam used for the recording and reproduction also has to be shortened.
In CDs and CD-ROMs which are currently employed, Al is coated on the concave and convex portions of the substrate therefor and the wavelength-dependence of the reflectivity of Al is so small that even if the wavelength of the laser beam used therefor is shortened in the future, reproduction of information recorded in such CDs and CD-ROMs is possible.
However, the recording layer of the currently employed CD-R, which uses therein a dye with a maximum absorption wavelength in the range of 680 nm to 750 nm, is designed so as to exhibit a maximum reflectivity when a laser beam with a wavelength of 770 nm to 750 nm is applied thereto, with the optical characteristics and constants and the thickness and structure of the recording layer taken into consideration.
Therefore, when a laser beam with a wavelength of 700 nm or less is applied thereto, the reflectivity exhibited by the recording layer is extremely small, so that the currently employed CD-R cannot cope with the shortening of the wavelength of the employed laser beam in the future. As a result, it will be highly possible that the information recorded by the currently employed CD-R system cannot be reproduced by such a future system.
Many recording materials, for example, cyanine dye/metal reflection layer, phthalocyanine dye/metal reflection layer, and azo metal chelate dye/metal reflection layer, have been proposed for use in the conventional CD-R systems.
More specifically, CD-R systems using as a recording material in the form of a cyanine dye/metal reflection layer are proposed in Japanese Laid-Open Patent Applications 1-159842, 2-42652, 2-13656, and 2-168446; CD-R systems using a phthalocyane dye as a recording material are proposed in Japanese Laid-Open Patent Applications 1-176585, 3-215466, 4-113886, 4-226390, 5-1272, 5-171052, 5-116456, 5-69860 and 5-139044; and CD-R systems using an azo metal chelate dye are proposed in Japanese Laid-Open Patent Applications 4-46186, 4-141489, 4-361088, and 5-279580.
However, there have not yet been proposed a CD-R system which solves the above-mentioned problems of the conventional CD-R systems.